The present invention relates to a method for forming an isolated film on a semiconductor device in the shape of a cylinder within a substrate by using a complex mask to shorten a heat treatment process and to prevent a micro-loading effect of filling of an oxide film.
In order to fully appreciate the scope of the present invention, the conventional fabrication process used in forming an isolated film on a semiconductor device according to the prior art will be described in more detail with reference to accompanying FIGS. 1A through 1F.
As shown in FIG. 1A, a pad oxide film 12 and a silicon nitride film 13 are formed on the Si substrate 11 in order. Nitride film 13 and pad oxide layer 12 are etched using a photoetching method by using the photosensitive material 14 to form a trench window, and then the Si substrate 11 is etched to form a trench and the remaining photosensitive material 14 is removed.
As shown in FIG. 1B, a p-type polysilicon layer 15, which functions as a channel stop diffusion source, is then deposited on the whole surface.
At this time, a BSG layer can be used for a channel stop diffusion source. Then, as shown in FIG. 1C, photosensitive material 16 is coated and etched to remove only a portion of the trench and an n-type well is formed through a photoetching process. The p-type polysilicon layer 15 is etched to remove only a portion thereof from the area at which the n-type well is formed and the photosensitive material 16 is then removed.
As shown in FIG. 1D, boron ions are diffused to form a field channel stop layer 18.
Furthermore, as shown in FIG. 1E, the p-type polysilicon layer 15 is removed and then a thermal oxide thin film 19 is formed within the formed trench. A CVD oxide film 20 is formed on the whole surface and a polymer 21 is coated on the concave portion to level the surface of the CVD oxide film 20.
Subsequently, as shown in FIG. 1F, the CVD oxide film 20 is dry etched to the surface of the nitride film 13 to form a semiconductor device on which the surface of the isolated film is leveled.
However, in the above-mentioned conventional technique for forming an isolated film, because a Si substrate is vertically etched in order to form a trench, a crystal defect is generated in the side of the trench formed within the Si substrate in forming a CVD oxide film.
Besides, when the polymer is coated in the concave portion of the CVD oxide film and the CVD oxide film is then etched in order to level the surface of the isolated film, if a difference in the pattern size of the isolated region will be generated, grooving of a field-isolated oxide is generated by the loading effect in a comparatively large pattern.